Electrochemical energy storage and conversion devices including batteries, capacitors and fuel cells have found great utility in power sources for portable electronics, energy storage stations and electric vehicles. For example, the Li-ion battery is a device which has high energy/power density. The battery includes positive electrodes and negative electrodes that are electrically insulated by a porous separator. Electrolyte is filled in the pores of the electrodes and separators to ionically connect the two electrodes. The positive electrode and negative electrode are composed of lithium active solid particles which are held to a thin metal sheet by binders. The thin metal sheet is called a current collector. The binder is typically a polymer that provides adhesion of the solid particles to each other and to the current collector.
Li-ion batteries have received extensive research and development efforts for decades, which have resulted in continuous performance, durability, and safety advancements of this technology. However, further improvement in energy and power density and reduction in cost are needed to compete with the internal combustion engine driving range. In a Li-ion cell, both “active” and “inactive” components are used, and the “active” materials are compounds that can intercalate/de-intercalate lithium ions during charge and discharge. The active materials are typically lithium transition metal oxides for positive electrode and graphite for negative electrode. during charge and discharge. Other components in the complex Li-ion cell system are referred to as “inactive” materials because they do not contribute to the capacity of the cell. Decreasing the fraction of inactive components in Li-ion cells is a straightforward approach for much higher energy density. Electrode thickness and porosity are the critical engineering parameters that significantly influence overall battery performance. Thick positive electrodes of 100-200 μm are required for high-energy-density applications.
The electrode is manufactured via a slurry mixing, casting and drying processes. The solid particles and binder are dispersed into solvent such as water or N-Methyl-2-pyrrolidone (NMP) to form a semiliquid mixture (slurry). The coating is produced by applying the slurry onto a current collector and drying out of the solvent. There is growing interest in fabricating composite cathodes through aqueous processing to make the process more environmentally benign. The reduction of cell cost is also of critical importance in cell design. Polyvinylidene difluoride (PVDF) is the most widely used binder for positive composite electrodes, which is dissolved in the volatile, expensive, and toxic NMP solvent during electrode manufacturing. Therefore, efforts have been made to study alternative binders which are water soluble, such as carboxymethyl cellulose (CMC), polyacrylic latex, and acrylate polyurethane. It has been shown that processing cost can be decreased significantly when water-based manufacturing is used in conjunction with thick electrode designs.
If water is used, however, the electrodes develop cracks during drying and have high residual stress when increasing coating thickness. These electrodes have poor performance when utilized in alkali-ion secondary (rechargeable) batteries. Cracking and residual stress in the coating is related to the build-up of capillary pressure during the drying process. The development of cracks during drying has been extensively reported in other particulate compositions such as desiccated soil, concrete casting, ceramic films and colloidal dispersions. It has been generally accepted that capillary stresses generated during drying are the cause. Stress induced cracking has been experimentally investigated by a cantilever technique. When a wet coating containing suspended particles is dried, the air-liquid interface reaches the sediment surface during drying. The meniscus of the air-solvent interface between particles generates a capillary pressure, and this pressure increases as solvent evaporates, which exerts further compression force on the particles. Eventually, the coating cracks at certain critical points to release the drying stresses. Several studies have also introduced numerical simulation into the dynamic observation of capillary shrinkage cracking. The results of these simulations indicate that crack initiation is due to capillary forces.
It has also been observed that cracking occurs only above a critical coating thickness. The critical thickness was found to be independent of the drying speed, and it actually increased with particle size because the capillary pressure scaled inversely with the particle radius.
Cracking has not drawn attention in aqueous processed coatings for Li-ion batteries. It is well-known that the surface tension of water (72.80 mN/m, 20° C.) is much higher than NMP (40.79 mN/m, 20° C.). Therefore, higher capillary pressure is expected in the drying processes of aqueous slurries that would lead to coating cracking.